To average user this means there is a second file (device tree blob) that needs to be loaded by the bootloader and handed over to the kernel. With overlays you shouldn't need to blacklist and modprobe kernel modules by hand depending what add-on boards you have attached to the rpi - now you just uncomment a line in config.txt that is required by your configuration.

Rootfs tarball is updated regularly regardless so you have a more current system of you are installing from scratch. If you already had arch installed from before pacman -Suy will bring it up to date - but you already knew that